Captain Moussa 'Dadis' Camara seized power in a coup in December 2008. Photograph: Schalk Van Zuydam/AP

The EU today imposed an arms embargo and a visa ban on Guinea's military leaders as new details emerged of killings and rapes allegedly committed by presidential troops at a peaceful rally last month.

The union is drawing up a list of 42 Guinean leaders who will be subject to the visa ban and plans to freeze assets of junta officials.

Human Rights Watch accused members of the presidential guard of carrying out a premeditated massacre of at least 150 people at the rally and raping dozens of women.

"Security forces surrounded and blockaded the stadium, then stormed in and fired at protesters in cold blood until they ran out of bullets," said Georgette Gagnon, the group's Africa director. "They carried out grisly gang rapes and murders of women in full sight of the commanders. That's no accident."

Captain Moussa "Dadis" Camara seized power in a coup in the impoverished west African state last December. After initially saying that he would not run in next January's elections, he recently indicated that he may have changed his mind.

During the peaceful pro-democracy rally on 28 September, presidential guard troops with their trademark red berets opened fire on tens of thousands of demonstrators, drawing condemnation from the West African regional group, Ecowas, the UN and the EU.

In its report today, Human Rights Watch said Camara and some of his closest military associates in the National Campaign for Democracy and Development (CNDD) junta should face criminal prosecution for the massacre.

"Many Guineans expressed shock at the apparent ethnic nature of the violence, which threatens to destabilise the situation in Guinea further," the group said.

Most of the victims were from the Peuhl ethnic group, which is mainly Muslim, while most of the commanders at the stadium and key junta members, including Camara, belong to ethnic groups that are largely Christian or animist.

A 42-year-old woman told Human Rights Watch of her ordeal: "One of them had a little container of white powder. He dipped his finger in it and forced it into my nose. Then all three of them used me. They used me again the next day, but after a while others came in, two by two. I didn't know how many or who. I felt my vagina was burning and bruised. I was so tired and out of my head. The first three of them were watching each other as they raped me."

Camara and the CNDD took control of the country after the death of President Lansana Contte, whose dictatorial rule lasted from 1984 to 2008. Initial popular support for the CNDD's anti-corruption campaign has given way to dissatisfaction over their unwillingness to commit to give way to civilian rule, and there are fears that Guinea's troubles could spill over into neighbouring countries. The border area with Liberia, which suffered a spillover from the Liberian civil war in 2001, is already experiencing increasing ethnic tension.